Cardiac magnetic resonance imaging (cardiac MRI, CMR), also known as cardiovascular MRI, is a magnetic resonance imaging (MRI) technology used for non-invasive assessment of the function and structure of the cardiovascular system. Conditions in which it is performed include congenital heart disease, cardiomyopathies and valvular heart disease, diseases of the aorta such as dissection, aneurysm and coarctation, coronary heart disease. It can also be used to look at pulmonary veins. Patient information may be found here.
It is contraindicated if there are some implanted metal or electronic devices such as some intracerebral clips or claustrophobia. These can be looked up to see if they are MRI conditional. For pacemaker or defibrillator patients, almost all can be scanned but special protocols are needed.
Conventional MRI sequences are adapted for cardiac imaging by using ECG gating and high temporal resolution protocols. The development of cardiac MRI is an active field of research and continues to see a rapid expansion of new and emerging techniques.
Uses
Cardiovascular MRI is complementary to other imaging techniques, such as echocardiography, cardiac CT, and nuclear medicine. The technique has a key role in evidence-based diagnosis and treatment of cardiovascular disease. Its applications include assessment of myocardial ischemia and viability, cardiomyopathies, myocarditis, iron overload, vascular diseases, and congenital heart disease. It is the reference standard for the assessment of cardiac structure and function, and is valuable for diagnosis and surgical planning in complex congenital heart disease.
Combined with vasodilator stress, it has a role in detecting and characterizing myocardial ischemia due to disease affecting the epicardial vessels and microvasculature. Late gadolinium enhancement (LGE) and T1 mapping allow infarction and fibrosis to be identified for characterizing cardiomyopathy and assessing viability. Magnetic resonance angiography may be performed with or without contrast medium and is used to assess congenital or acquired abnormalities of the coronary arteries and great vessels.
Obstacles to its wider application include limited access to scanners, lack of technologists and skilled clinicians, relatively high costs, and competing diagnostic modalities. Some organizations are working on solutions to reduce these obstacles so that more clinics can adopt CMR into their practices. These solutions are often software platforms that provide clinical decision support and improve the efficiency of the procedures.
Risks
Cardiac MRI does not pose any specific risks compared to other indications for imaging. Gadolinium based contrast medium is frequently used in CMR and has been associated with nephrogenic systemic fibrosis, predominantly using linear compounds in patients with renal disease. More recently evidence of intra-cranial deposition of gadolinium has been shown - although no neurological effects have been reported. Genotoxic effects of cardiac MRI have been reported in vivo and in vitro, but these findings have not been replicated by more recent studies, and are unlikely to produce the complex DNA damage associated with ionizing radiation.
Physics
CMR uses the same basic principles as other MRI techniques. Imaging of the cardiovascular system is usually performed with cardiac gating using an adaptation of conventional ECG techniques. Cine sequences of the heart are acquired using balanced steady state free precession (bSSFP) which has good temporal resolution and intrinsic image contrast. T1-weighted sequences are used to visualize anatomy and detect the presence of intra-myocardial fat. T1 mapping has also been developed to quantify diffuse myocardial fibrosis. T2-weighted imaging is mainly used to detect myocardial edema which may develop in acute myocarditis or infarction. Phase-contrast imaging uses bipolar gradients to encode velocity in a given direction and is used to assess valve disease and quantify shunts.
Techniques
A CMR study typically comprises a set of sequences in a protocol tailored to the specific indication for the exam. A study begins with localisers to assist with image planning, and then a set of retrospectively-gated cine sequences to assess biventricular function in standard orientations. Contrast medium is given intravenously to assess myocardial perfusion and LGE. Phase contrast imaging may be used to quantify valvular regurgitant fraction and shunt volume. Additional sequences may include T1 and T2-weighted imaging and MR angiography. Examples are below: